Delignification of lignocellulosic materials with a solution of nitrogen dioxide in aliphatic hydrocarbon solvent

ABSTRACT

A INVENTION RELATES TO THE DELIGNIFICATION OF CELLULOSIC MATERIALS WHICH ARE, IN A FIRST STEP, SUBJECTED TO THE ACTION OF NITROGEN DIOXIDE IN AN ALIPHATIC HYDROCARBON SOLVENT TO PRODUCE REACTION PRODUCTS OF NITROGEN DIOXIDE AND THE NONCARBOHYDRATE COMPONENTS. AFTER SEPARATION OF THE UNUSED NO2 REAGENT AND RESIDUAL SOLVENT, THE TREATED MATERIALS ARE EXTRACTED WITH A WEAK ALKALINE SOLUTION TO REMOVE THE LIGNIN RENDERED SOLUBLE BY THE FIRST STEP.

Feb. 6, 1973 F. BENDER ETAL 3,715,268

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3,15,268 EATERIALS WITH A SOLUTION Feb. 6, 1973 .BENDER ETAL DELIGNIFICATION OF lJIGNOCEbLUhOSIC OF NITROGEN DIOXIDE IN ALIPHATIC HYDROCARBON SOLVENT Filed Oct. 22. 1970 3 Sheets-Sheet Z OON njna mwz 256 4 1 01 53a 51 5 056459 54 352,20 4 No.4 flmbzwum O :Vuzwukw 555 6:5 6 mmmzmwmu 00 8m 8 8 -m-d) HLEDHQZLC; L zmq IN VENTOR 5 mom flowzmma 0 IPOZME 5.015. A015 0.0 mmwzwumu 53a m z 2504 056459 4 OOmOOQOQ PA TENT AGENT Feb. 6, 1973 BENDER ETAL 3,715,268

DELIGNIFICATION OF LIGNOCELLULOSIC MATERIALS WITH A SOLUTION OF NITROGEN DIOXIDE IN ALIPHAIIC HYDROCARBON SOLVENT Filed 00$. 22, 1970 3 Sheets-Sheet 5 (upul/Qm) HLDHQZILQ Q'HQ-HQL 1'1 5 /43/11- A I/K im-41; 1- 30 Z PA TENT AGENT United States Patent 3,715,268 DELIGNIFICATION OF LIGNOCELLULOSIC MATE- RIALS WITH A SOLUTION OF NITROGEN DIOXIDE IN ALIPHATIC HYDROCARBON SOLVENT Frederick Bender and Louis-Philippe Clermont, Ottawa, Ontario, and Alan Warren Bowden, Prince Rupert,

Canada Filed Oct. 22, 1970, Ser. No. 81,990 Claims priority, application Canada, Oct. 24, 1969,

8 rm. (:1. D21 c 3/16, 3/20 US. Cl. 162-72 11 Claims ABSTRACT OF THE DISCLOSURE The present invention relates generally to the production of pulp from lignocellulosic materials.

The aim in the production of chemical and semi-chemical pulp from lignocellulosic materials, such as wood and straw, is to solubilize a part of the starting material and, by doing so, to transform the undissolved part either into pulp or into a material which can be readily defibered by mechanical means, such as disc refiners. Many inorganic and organic reagents are available which can be used for this purpose. Since it is, in some cases, difficult to achieve a sufiicient degree of delignification in one step, several steps might be applied in succession. However, each of these steps individually is a pulping process based on the concept of selective solubilization of the moncarbohydrate components of the raw material.

The existing pulping processes of industrial importance are carried out at temperatures considerably above 100 C.; conditions of pH, chemicals concentration and temperature result in solutions of lignin degradation products which so far have found very limited use for chemical purposes. The end result is a pollution problem of great severity and a loss of large quantities of organic material.

The present invention resides in a two-step process, the first step of which comprises subjecting the lignocellulosic materials to the action of nitrogen dioxide in a hydrocarbon solvent to produce reaction products of nitrogen dioxide and the non-carbohydrate components of the materials, the reaction products being insoluble in the solvent. After separation of the unused N0 reagent and residual solvent from the treated chips, the latter, after a brief washing with water, are extracted with a weak alkaline solution to remove lignin rendered soluble by the first step.

The invention has the following advantages:

(1) Low cost of chemicals, since the solvent and part of the N0 can be recovered and the caustic of the second step can be regenerated.

(2) The lignin is recoverable in a reactive form and is consequently more valuable as a chemical raw material.

(3) Problems of air and water pollution are much re duced as compared with conventional processes.

(4) Costs of installation and operation can be kept low since no elevated pressures and very moderate temperatures are applied.

(5) The process is readily adaptable to continuous operation because of the high rate of reaction in each step.

In accordance with the invention, lignocellulosic materials are treated with a reagent comprising nitrogen dioxide dissolved in a hydrocarbon solvent which in itself dissolves neither lignin nor cellulose. The N0 transforms the noncarbohydrate part of the wood in situ without rendering it soluble in the reaction medium. The N0 is partly reduced to NO, which in the presence of air or 0 is readily reconverted to N02.

Hydrocarbon solvents, and particularly aliphatic hydrocarbons consisting mainly of parafiins and naphthenes and containing only small fractions of aromatics, are suitable media. The desirable boiling range is 100 C. to 200 C. Many petroleum distillates, which are commercially available, fall within this range. Kerosene, diesel oils and stove oils are examples. Other suitable petroleum distillates are those sold under the trade names Varsol and Iosol by Imperial Oil Limited. So long as the aromatics content is very low, the amount of reaction between N0 and the solvent at room temperature or slightly elevated temperatures is small.

Solutions of N0 in the mentioned solvents react with wood at room temperature without dissolving the reacted material. Therefore, this step of the process can be effectively carried out at room or slightly elevated temperatures of, say, 20 C. to 50 C.

Solutions of 0.5 to 3% N0 are satisfactory for the purpose, (0.5-3 grns. N0 per 100 ml. of solution). A 2% solution has been found to be quite effective. The weight of N0 present, based on the weight of dry wood, desirably varies from about 10 to 30% The lignocellulosic material, such as a body of wood chips, is impregnated with a solution as described. The impregnation step may be conducted under vacuum. Preferred liquor to wood ratios are from about 5:1 to 10: 1.

Following impregnation with the solution, the treated material is allowed to stand to permit the ensuing reaction to take place. However, this reaction takes place rapidly and usually within a period of 10 to 60 minutes.

After the reaction has taken place, the solvent and any surplus reagent are recovered, and the treated material remaining is washed with water and then subjected to the second step of the process, which consists of an extraction with dilute aqueous alkali such as NaOH. In the extraction step, temperatures below 100 C. and an alkali concentration not substantially greater than 1% have been found to be sufiicient to remove the reacted lignin of the first step. The amount of alkali present based on the weight of dry wood is desirably about l0%15%.

Pulp yields in the range of 50 to based on the dry wood, can be produced, depending upon the conditions chosen.

The alkali extracts contain predominantly lignin fractions which are characterized by a considerable degree of reactivity. Upon acidification or causticization, most of the lignin precipitates and can be easily separated.

In some instances, the lignocellulosic material, such as wood chips, may be subjected to a pretreatment with steam. Both broad-leafed and coniferous woods can be treated successfully.

The invention is further illustrated with reference to the following examples and with reference to the accompanying drawings, in which:

FIG. 1 is a diagram showing curves of beating times vs. freeness for treated aspen chips,

FIG. 2 is a diagram showing curves of beating times vs. freeness for treated black spruce chips,

FIG. 3 is a diagram showing comparative curves of tensile strength vs. freeness for a commercial pulp and an aspen pulp prepared in accordance with the invention,

FIG. 4 is a diagram showing comparative curves of burst strength vs. freeness for a commercial pulp and an aspen pulp prepared in accordance with the invention,

FIG. 5 is a diagram showing comparative curves of burst strength vs. freeness for a commercial pulp and a black spruce pulp prepared in accordance with the invention, and

FIG. 6 is a diagram showing comparative curves of tensile strength vs. freeness for a commercial pulp and a black spruce pulp prepared in accordance with the invention.

EXAMPLE I Aspen chips (1-2 mm. thick and 2-2.5 cm. long) with approximately 8% moisture content are vacuum impregnated with 2% (w./v.) solution of NO; in coal oil (20% of o.d. wood). After 30 minutes standing at room temperature, the coal oil-N0 solution is drained off and residual chemical and solution are removed by blowing first air and then steam through the reaction vessel for 15 minutes. After washing with water, the treated chips are extracted with a 1% NaOH solution (10% of o.d. wood) at 90 C. for 30 minutes and passed twice through a 12" Sprout-Waldron laboratory refiner. The resulting pulp (75% yield) has the following characteristics:

Copper number 1.1

Chlorine number 18.1

Pentosans "percent" 15.1

Alpha cellulose do 65 EXAMPLE II Air-dried spruce chips are vacuum impregnated with a 2% N0 (30% of o.d. wood) solution in coal oil and left standing for 60 minutes at room temperature. After this time the solution is drained off and any residual solvent and chemical removed by blowing steam through the reaction vessel. After washing with water the treated chips are extracted twice for 30 minutes with 1% NaOI-I (15% of o.d. wood). Thereafter, they are passed twice through a 12" Sprout-Waldron laboratory disc refiner. The resulting pulp (77.3% yield) has a chlorine number of 29.2 and a copper number of 1.3.

EXAMPLE III Air-dried aspen sawdust, treated as in Example 1, yielded 52% pulp with a chlorine number of 1.7 and a copper number of 1.6.

EXAMPLE IV Air-dried black spruce sawdust treated as in Example II yielded 70% pulp with a copper number of 1.8 and a chlorine number of 9.9.

All pulps had degrees of polymerization over 1000.

EXAMPLE V Aspen chips are steamed for minutes and treated as in Example I. The pulp yield is 67.0 copper number of 1.0 and chlorine number of 10.6. This steam treatment decreases the yield, the chlorine number and the screening rejects of the pulp as compared to air-dried chips.

EXAMPLE VI Black spruce chips are steamed for 20 minutes and treated as in Example -II. The pulp yield is 68.3% copper number 0.8, chlorine number 19.4 and screening rejects are 5.2% as compared to 13.5 for air-dried chips.

EXAMPLE VII Wet aspen chips (moisture content approximately 35%) are treated as in Example I. The pulp yield is 75.6%, chlorine number 11.7%, screening rejects 3.7%. Both figures are lower than for air-dried chips.

EXAMPIJE VIII Wet black spruce chips (moisture content approximately 35%) are treated as in Example II. The yield is approximately the same as for air-dried chips (79.2%.) but the chlorine number (21.8) is lower and the screening rejects (3.9%) are also lower than with air-dried chips.

EXAMPLE IX Aspen veneer chips (0.55 mm. thick, 1 cm. long) containing 7.9% moisture are treated for one hour at room temperature with a solution containing 2% N0 (w./v.) in a commercial hydrocarbon solvent (Varsol) at a 10:1 liquor to wood ratio. The solvent and excess N0 are removed by steam treatment for 15 minutes. The steamed chips are washed with water to pH 5 and extracted with 1% NaOH for 30 minutes at to C. with direct steam injection. The pulp yield is 60.9%, Klason lignin 7.0% and copper number 0.57.

EXAMPLE X Aspen veneer chips, containing 20% moisture, are treated as in Example IX. The pulp yield is 48.9% Klason lignin: 0.62%, copper number: 1.58; nitrate DR: 1215.

EXAMPLE XI Aspen veneer chips (16% moisture) are treated as in Example IX, except that the treatment time with N0 was 10 minutes. The alkali extraction was carried out for 1 hour at 90 C. The pulp yield was 60.1%, the chlorine number 3.6.

EXAMPLE XII Aspen veneer chips are steamed for 15 minutes and treated as in Example IX. The pulp yield is 55.7%, the Klason lignin 5.4%, the copper number 0.85 and nitrate D.P. 1749'.

EXAMPLE XIII Aspen veneer chips containing 50% moisture are treated for /2 hour as in Example IX. The pulp yield is 57.5%, the Klason lignin 1.7%, the copper number 0.35.

EXAMPLE XIV Black spruce veneer chips (0.3 mm. thick, 2 cm. long), moisture content 26.6% are treated for one hour as in Example IX. The pulp yield is 64.1%, the Klason lignin 13.1%.

EXAMPLE XV Black spruce veneer chips containing 26.6% moisture are treated with a solution of 3% N0 (30% of o.d. wood) in Varsol as in Example IX. The pulp yield is 54.4%, the Klason lignin 0.8%.

Beater curves for aspen chips treated as in Example IX and for black spruce chips treated as in Example XV are given in FIGS. 1 and 2. These chips were defibered in a Sprout-Waldron disc defibrator prior to the beater tests, which may explain their low initial freeness. All these pulps are fast beating.

Burst and tensile strength curves for aspen and spruce pulps prepared as in Examples IX and XV and bleached by the C-E-D-E-D sequence are shown in FIGS. 3-6. These chips have an initial moisture content of 16.0%. A commercial bleached sulfite pulp has been included for comparison purposes.

We claim:

1. A process for delignifying lignocellulosic materials which comprises subjecting a body of lignocellulosic material to the action of a solution of nitrogen dioxide in an aliphatic hydrocarbon solvent to impregnate said material therewith, at a temperature in the range 2050 C., for sufficient time to modify the non-carbohydrate components of said material with nitrogen dioxide, said modified components being insoluble in said solvent, separating residual reaction solvent medium from the treated material including said modified components and subject- 1 ing the treated material including said modified components to an alkaline extraction treatment thereby separating said modified components from said cellulosic material.

2. A process for delignifying lignocellulosic materials as defined in claim 1, wherein said solvent is a mixture of hydrocarbons.

3. A process for delignifying lignocellulosic materials as defined in claim 2, wherein said solvent is a predomi nately aliphatic petroleum distillate.

4. A process for delignifying lignocellulosic materials as defined in claim 1, wherein said solution has a concentration of 0.5 to 3% N 5. A process for delignifying lignocellulosic materials as defined in claim 1, wherein said time is 10 to 60 minutes.

6. A process for delignifying lignocellulosic materials as defined in claim 1, wherein said action is at room temperature.

7. A process for delignifying lignocellulosic materials as defined in claim 1, wherein said body of material is pretreated with steam prior to said impregnation step.

8. A process for delignifying lignocellulosic materials as defined in claim 1, wherein said alkaline extraction step is conducted at an alkali concentration not substantially exceeding 2% at a temperature not substantially exceeding 100 C.

9. A process for delignifying lignocellulosic materials as defined in claim 1, wherein said alkaline reaction step is conducted with a sodium hydroxide solution of a concentration of about 1% and at a temperature of 85- 100 C.

10. A process for delignifying lignocellulosic materials as defined in claim 1, including the step of recovering said residual solution and regenerating the nitrogen dioxide with oxygen or air.

11. A process for delignifying lignocellulosic materials as defined in claim 1, wherein said impregnation step is conducted under vacuum.

References Cited UNITED STATES PATENTS S. LEON BASHOR'E, Primary Examiner A. L. CORBIN, Assistant Examiner U.S. Cl. X.R. l6281 

